This invention relates to power tongs used to turn pipe in the oilfield industry. More particularly, it relates to a camming system by which the jaws within a power tong are caused to engage with pipe.
In the oil industry power tongs are used to either grasp and hold pipe, tubing and casing while threaded joints are being made up or broken, or to rotate pipe, tubing and casing in order to make up or break such joints and to effect drilling. In the past, power tongs have been provided with at least one jaw mounted in a displaceable jaw assembly that may be shifted in order to bring the jaw into bearing engagement against pipe. Jaw systems of this design are typically constrained by upper and lower cage plates to which the jaw assembly is connected.
Typically, one or more jaws are caused to advance inwardly by rotating an outer encircling camming surface about the jaw assembly with the pipe located at the center of rotation. This camming surface is typically formed on the inside of a C-shaped ring gear. Camming rollers on the jaw assembly act as a cam follower bearing against this camming surface, and causing the jaw assembly to advance inwardly in response to the inward urging developed by the cam surface as it rotates past the camming rollers. The jaws in many typical designs pivot about a pivot axis that is fixed in relationship to the cage plates, the path followed by the jaws as they are displaced inwardly being arcuate. In other jaw designs the jaws advance radially towards the center of the tong in a sliding action.
Once the jaws have contacted the pipe, a very high level of radial force be applied to the pipe in order to ensure a non-slipping, frictional engagement persists while torque is applied to the pipe. High torque forces are required in order to ensure that the joints in the drill pipe are properly made up, and to turn the bit mounted at the bottom of the drill pipe string where the boring of the earth is occurring.
In order to torque pipe with minimal damage to the pipe surfaces very high radial forces are required when the jaws are equipped with smooth pipe-engagement surfaces. Smooth-faced gripping jaws are particularly employed with casing and tubing that are made of exotic alloys to reduce corrosion. Such high radial forces are achieved by providing a camming surface with a very gradual inwardly-directed slope along which the camming roller is required to advance. Consequently, in order to maximize the radial forces that can be applied to the pipe, it is important that the camming roller should be able to advance along the camming surface with a minimum level of frictional resistance.
An example of a prior art patent addressing this issue is U.S. Patent 5,819,605 to Buck for xe2x80x9cLow Friction Power Tong Jaw Assemblyxe2x80x9d. In this patent it is proposed to provide a friction reducing surface between a jaw roller and the surface of a roller retaining means that secures the jaw roller to an aperture in the body of the jaw assembly. Examples given include pin, ball or sleeve bearings used to support the jaw roller. However, in every case depicted, only a single roller is in contact with the camming surface. Contact occurs along a single line contact that is limited to the length of a single roller.
It would be advantageous to provide a power tong wherein the frictional resistance encountered between the jaw holder and the camming surface is reduced to a low-level. The present invention addresses that objective.
The invention in its general form will first be described, and then its implementation in terms of specific embodiments will be detailed with reference to the drawings following hereafter. These embodiments are intended to demonstrate the principle of the invention, and the manner of its implementation. The invention in its broadest and more specific forms will then be further described, and defined, in each of the individual claims which conclude this Specification.
The present invention in one aspect is applicable to a power tong provided with a powered rotary ring gear having a central pipe-containing region accessible through a throat opening in the side of the ring gear. The inner surface of the ring gear includes a generally crescent-shaped camming surface or surfaces that is/are of a minimum height adjacent to the throat, rising towards a maximum, inwardly-directed height proceeding towards the inside of the ring gear opposite to the throat.
Mounted above and below the ring gear are twin C-shaped cage plates each with a key-shaped gap. The gap in the xe2x80x9cCxe2x80x9d of each cage plate is comparable in size and corresponds with the central pipe containing region and throat opening in the ring gear when aligned. These cage plates are free to rotate with respect to the ring gear, but are constrained to maintain a center of rotation corresponding to the center of the ring gear and the drill pipe to be contained therein.
According to a preferred embodiment, a first, receiving jaw means is mounted between the two cage plates. The receiving jaw means is preferably in the form of a pair of pipe receiving jaws that are pivotally mounted and positioned to receive and self-center pipe that has passed through the throat in the ring gear and gaps in the cage plates to be positioned centrally within the rotary gear. While this is a preferred arrangement, any system that positions the pipe to be turned and assists in the engagement of the pipe may be employed as the receiving jaw means. This may include a second or further jaw assembly as next described below.
At least one jaw assembly incorporating a pipe-engaging jaw is also carried between the upper and lower cage plates, mounted to allow the jaw assembly to advance inwardly between the plates. With the pipe centered in place, upon rotation of the ring gear, (with the cage plate restrained), the jaw assembly is placed into alignment with the beginning of the camming surface on the ring gear. A cam follower bearing means according to the invention is mounted within the jaw assembly, positioned to contact and advance along the camming surface. Rotation of the ring gear forces the jaw within the jaw assembly to move inwardly in response to the inwardly-directed force developed by the camming surface on the cam follower. Such action continues as the ring gear is rotated until the jaw within the jaw assembly contacts the pipe. This occurs before the cam follower reaches the maximum height of the camming surface within the ring gear.
The inwardly directed force developed by the camming surface on the cam follower bearing means, or xe2x80x9ccam followerxe2x80x9d, then causes the pipe-contacting die face of the jaw to bear tightly against the pipe. Once this tight engagement is achieved, the pipe will be forced to rotate with the ring gear. The cage plates and jaw assembly, together with the receiving jaw means, will then rotate with the pipe as well.
The cam follower of the invention is characterized by a plurality of circulating roller elements, preferably cylindrical roller bearings, that are constrained to circulate through a roller bearing passageway formed in association with the body of the jaw assembly. During a portion of their circulation, the roller elements are contained within an inner portion the roller bearing passageway while passing through the body of the jaw assembly. Their circulating path then continues along an outer bearing face of the jaw assembly, exposing the roller elements so that they are directed towards the camming surface. Further, the shape of the bearing face of the jaw assembly is curved to ensure that at least two and preferably four or more roller elements are in contact with the camming surface, in the xe2x80x9ccamming zonexe2x80x9d of the camming surface.
The roller elements are preferably contained while on this outward portion of their path by upper and lower containment means. Such containment means may be in the form of grooved tracks set into the surfaces of containment plates The roller elements, in the case of cylindrical bearings, are conveniently maintained in parallel alignment by being tightly packed. Alternately spacers may be present between such cylindrical bearings to maintain them in parallel alignment, oriented transversely across the circulating path along which such bearings advance.
The features of the invention as described above may be incorporated into a power tong wherein the jaw assembly is mounted for pivoting advancement towards a center of the power tong. Alternately, the jaw assembly may be mounted for radial advancement towards the center of the tong. In either case, the use of multiple camming roller elements that are pressed against the camming surface while traveling in a circulating path associated with the jaw assembly serves to greatly reduce the frictional resistance to the advancement of such cam follower roller elements along the camming surface when the tong is applying torque and consequently developing high radial forces to ensure that an effective grip is maintained on the pipe.
While described in the context of a single jaw and jaw assembly being advanced inwardly, the invention may also be employed with two or more jaws having a similar cam follower system to that described, wherein the camming roller elements of each jaw assembly are each caused to advance along a corresponding camming surface within the ring gear while circulating within their respective jaw assembly.
According to a further feature of the invention, the jaw assembly is introduced into the ring gear through the throat of the tong. In this variant the jaw assembly is initially stored at a storage location on the inside face of the gate used to close the tong throat. Upon closing of this gate, the jaw assembly may be advanced by sliding it inwardly through the throat to a position within the ring gear. Thereafter, rotation of the ring gear with respect to the jaw assembly commences the camming action.
The foregoing summarizes the principal features of the invention and some of its optional aspects. The invention may be further understood by the description of the preferred embodiments, in conjunction with the drawings, which now follow.